University of Leicester scientists reveal first light from stellar collision discovered by gravitational waves

‘This discovery fundamentally changes how we do astronomy’ --Professor Paul O’Brien, University of Leicester

‘This first event is already making shockwaves’ - Dr Phil Evans, University of Leicester

‘I find it amazing to think that all the gold in the Earth was probably produced by merging neutron stars, similar to this one, that exploded as kilonovae billions of years ago’ - Professor Nial Tanvir, University of Leicester

‘This first joint detection of light and gravitational waves from the collision of two neutron stars is a key milestone result in astrophysics’ - Professor Julian Osborne, University of Leicester

Scientists from the University of Leicester have today published the first detections of light accompanying gravitational waves from colliding neutron stars.

Teams led by University astronomers have explored for the first time the detailed aftermath of the collision of two neutron stars: a violent interaction which produces the heaviest chemical elements in the universe.

Today the advanced LIGO and Virgo gravitational wave observatories announced details of the event which occurred on 2017 August 17th. The gravitational wave signal was accompanied by gamma rays seen by the Fermi satellite, the first time that light has been definitively detected from a gravitational wave source. Astronomers across the world began searching for the precise location of this event, quickly tracking it down to the nearby galaxy NGC 4993. University of Leicester astronomers using the VISTA telescope in Chile were among the first to locate the new source. “Our strategy was to hunt through many galaxies, searching for evidence of an ongoing explosion”, explained Prof Nial Tanvir, who leads a paper in Astrophysical Journal Letters today.

Once pin-pointed, the Swift satellite quickly maneuvered to look at the object with its X-ray and UV/optical telescopes. “We didn’t detect any X-rays from the object, which was surprising given the gamma ray detection,” said Dr Phil Evans, lead-author of a paper published today in Science. “But we did find bright ultra-violet emission, which most people were not expecting. This discovery helped us to pin down what happened after the neutron star collision that LIGO and Virgo saw.”

Neutron stars are the dead remnants of massive stars, they contain the mass of the sun in an object the size of a city. When they collide, some of the neutrons are ripped off and start to interact with each other, forming some of the heaviest elements in the universe. Radioactive decay of these elements then produces light in what is often called a ‘kilonova’. “The exact origin of these heavy elements, including gold and platinum, has been debated for decades” said Prof Tanvir. “To finally observe a kilonova in great detail, to see how it evolves and changes colour allows us to confirm that colliding neutron stars really are an important source of these elements."

The Swift data gave unprecedented insight into how this nucleosynthesis occurs after a neutron star merger. “We found that there were two sites where this took place,” explained Dr Evans. “We expected slowly rising red emission from where the heaviest elements are created. The Swift data showed that there was also early ultra-violet and blue emission, in a pulse lasting just one day, which came from material blown away in a wind. This does not make the heaviest elements, so the blue light can escape. The discovery of the neutron wind was only possible using light, which is why combining gravitational waves and light in what we call ‘multi-messenger astronomy’ is so important.”

University of Leicester astronomers also studied the polarisation of the light using the Very Large Telescope in Chile. “We found no sign of polarisation from the kilonova”, said Dr Klaas Wiersema, co-author of a study published in Nature Astronomy. “This suggests that there must have been a roughly spherical outflow of material after the merger, such as the wind that Swift detected.”

“This is a really exciting event” concludes Professor Tanvir, who also used the Hubble Space Telescope to study this object. “For the first time we have detected an event in both gravitational waves and light, which gives us excellent complementary information. This and future events will enable us to learn much more about neutron stars, black holes and physics in really extreme environments and the creation of heavy chemical elements in the universe.”

Further Information:

Swift was launched in November 2004 and is managed by NASA's Goddard Space Flight Center. Goddard operates the spacecraft in collaboration with Penn State University, the Los Alamos National Laboratory and Orbital Sciences Corp. The Swift project is an international collaboration between the USA, the United Kingdom and Italy.

The X-ray camera on Swift was designed and built at the University of Leicester. The University continues to provide calibration support for the X-ray telescope and runs the UK Swift Science Data Centre (http://www.swift.ac.uk) that makes Swift data and products available within hours of it being collected. The UV/optical telescope was constructed at the Mullard Space Science Laboratory, UCL. UK involvement in Swift is funded by the UK Space Agency.

Prof. Tanvir and Dr. Wiersema’s work was funded by the UK Science and Technology Facilities Council (STFC)

The VLT and VISTA telescopes are part of the European Southern Observatory (ESO), of which the UK is a member state through STFC.

This discovery fundamentally changes how we do astronomy and is a tremendous achievement. I am involved in designing the observations we make and I am the University liaison with the LIGO/VIRGO team. I am also working on future ground and space facilities to expand our ability to quickly find the light from gravitational wave sources. This event is just the start of the new era.

In our discipline, the study of high energy explosions in the universe, this is the discovery of the decade. It may be a cliché to say it, but truly a new window on the universe has opened.

I am very excited about this discovery. I didn’t think an optical counterpart to a gravitational wave event would be found so early on in the operation of Advanced-LIGO. We now have years of exciting research ahead of us, and I can’t wait to get stuck in!

Together with my co-Principal Investigator, Professor Covino, I obtained polarisation observations with the Very Large Telescope in Chili. We believe this to be the best way to measure the shape of the kilonova, and we could finally test our theories now!

It was a tremendously thrilling few weeks of observing, getting data on a completely new and unexplored phenomenon.

At the University of Leicester, PhD students get involved in front line research, which you can see from our publications on this source: our PhD students played an important role.

We can now study the violent energetic processes in the merger of two neutron stars, using both visible light and gravitational wave information at the same time.

This gives us a frontline view of processes way out of reach of terrestrial laboratories, testing the laws of physics in the most extreme environments.

For the first time ever we’ve been able to combine the new astronomical tool of gravitational waves, with the traditional one of light. It allows us to study things in ways never before possible. It’s a bit like being a naturalist who initially can see but not hear; who then learns to hear but doesn’t know where the sounds come from, then one day hears sounds AND sees which animal made which noise! That’s what has happened for us, and it’s incredibly exciting to finally be able to study the cosmos in this combined way, and who can tell what we’re going to find? After all, we’ve never been able to look in this way before!

This first event is already making shockwaves. We expected that when two neutron stars merge, we’d see a short flash of gamma-ray, and see a glow of emission as nuclear reactions create chemicals like gold and platinum — and we did see both of those things. But neither of them looked like we expected, and we have a lot of work to do now to use these results to improve our understanding of physics. This is the sort of thing we can’t do in a lab - the conditions are too extreme — so this new ‘multi-messenger’ astronomy is the only way we probe this type of physics.

Of course, results like this don’t just happen; they come about after years of hard work by teams of people. I’ve spent many years finding ways to use the Swift satellite to look for the counterparts to gravitational waves, which is very difficult as we can’t tell very accurately where those waves came from. In this event, our early observations with Swift, in which we found no X-rays (which surprised us) but lots of bright ultra-violet emission (which really surprised us), were vital to understanding the nature of this object left behind after the neutron stars merged.

Professor Nial Tanvir:

I am the principal investigator for a large program (called VINROUGE) operating at the European Southern Observatory's VISTA telescope, dedicated to searching for kilonovae associated with gravitational wave detections. I am also principal investigator for a programme that used the Hubble Space Telescope to observe the explosion, as well as being involved in several other campaigns using various other powerful telescopes.

We were really excited when we first got notification that a neutron star merger had been detected by LIGO, and immediately triggered observations on several telescopes in Chile to search for the explosion that we expected it to produce. In the end we stayed up all night analysing the images as they came in, and it was remarkable how well the observations matched the theoretical predictions that had been made."

I find it amazing to think that all the gold in the Earth was probably produced by merging neutron stars, similar to this one, that exploded as kilonovae billions of years ago.

This discovery has opened up a new field of science that we call `multi-messenger' astronomy, where we use information gathered from both gravitational waves and normal electromagnetic light, to probe some of the most exotic and extreme events that happen anywhere in the universe.

This is a key breakthrough, since it is the first time ever that we've been able use both light and gravitational waves to study something -- in this case the merger of two neutron stars -- happening far away in the universe. It is also a major step forward in solving a puzzle that goes back more than half a century, namely the origin of the heavy elements.

I have to say, when we first located the kilonova explosion, and later as we followed its evolution it seemed almost surreal. We have been planning for years how we would make these observations given the chance, but I don't think anyone expected the first case to be so perfect. It did mean working round the clock for several weeks, but these opportunities don't come along very often, so it was worth it.

Without doubt, we now have a much clearer idea just how to move forward with this science in the future. We also have firm evidence that neutron star mergers do produce copious amounts of heavy elements, and so very likely similar mergers long-ago are the major source of all the gold and platinum that we find in the Earth.

Prof Julian Osborne:

This first joint detection of light and gravitational waves from the collision of two neutron stars is a key milestone result in astrophysics. After the first detection of gravitational waves from colliding black holes in 2015 opened up this new way of seeing the Universe, we have been hoping for a neutron star collision that would be simultaneously detectable with light. Gravitational waves give us unique information about the colliding stars, but to understand their local environment, their history and the collision aftermath we have to use all the information we can get from across the electromagnetic spectrum of light. This collision has presented some surprises, in its weak gamma-ray emission, and its initial ultra-violet pulse, suggesting an off-axis view of the binary orbital rotation axis. However, scientists love surprises, which lead to new knowledge, but also the great promise that this joint detection technique offers, with an increased rate expected in the future, in understanding these systems in general and for what may be learned about physics, cosmology and the more distant Universe.

Today, we are announcing the discovery, using the Swift satellite, of an pulse of ultra-violet light in the first day after the neutron star collision. With the longer-lasting and redder light also seen, we inferred two forms of ejecta. The red light comes from neutron-rich material spun out into the orbital plane of the binary, the very heavy elements present, such as gold and platinum, absorb any blue light that would be emitted. The ultra-violet/blue light emerges from a neutrino-irradiated wind in which the very heaviest elements have not formed, thus allowing the shorter wavelengths to escape. This work is published today in the high-profile journal Science.

Swift, launched in 2004, is a US/UK/Italian satellite for which the UK provided substantial elements of the X-ray and UV/optical telescopes. In the last few years the Swift team at the University of Leicester, which I lead, has been developing, with Penn State University in the USA, the new operational capabilities needed to rapidly cover the large sky areas identified by the gravitational wave instruments as the location of the collisions.